ML19325D187

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LER 88-026-03:on 881118,engineering Postulated Scenario Which Could Result in Redundant Trains of safety-related Equipment.On 881229,three Relays Identified W/Similar Problem.Caused by Design inadequacy.W/891010 Ltr
ML19325D187
Person / Time
Site: Millstone Dominion icon.png
Issue date: 10/10/1989
From: Keller R, Scace S
NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-88-026, MP-13604, NUDOCS 8910190151
Download: ML19325D187 (6)
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Text

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!' Nonneast Nuew energy conwany (203)666-6000  : I ' i Det.ober 10. 1989 j MP-1360!i , Re: 10CFR50.73(a)(2)(v)  ! F [. l U.S. Nuclear Regulatory Commission Document Control Desk ' Washington, D.C. 20555

Reference:

Facility Operatinc License No. NPF-49  ! Docket No. 50-4'23 i Licensee Event Report 88-026-03 , Gentlemen: f This letter forwards Licensee Event Report 88-026-03 submitted as an update to report i a sicnificant change made to the modification proposed in LER 88-0 6-02. LER's 88-626-00, 88-026-01 and 88-026-02 were submitted pursuant to 10CFR50.73(a)(2) t (v), any event or condition that alone could have prevented the fulfillment of the safety - 

       . function of systems that are needed to mitigate the consequences of an accident.                          ;

Very truly yours, > NORTHEAST NUCLEAR ENERGY COMPANY i A cact - Steph E. Scace Station Superintendent , Millstone Nuclear Power Station  ; SES/Rh%mo

Attachment:

LER 88-026-03 cc: W. T. Russell, Region 1 Administrator W. J. Raymond, Senior Resident inspector, Millstone Unit Nos.1, 2 and 3 D. H. Jaffe, NRC Project Manager, Millstone Unit No. .s [22-8910190151 891010 'I FDR ADOCK 05000423 5 PDC 

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F ACluTY NAME o) UQ4r L1 NsMbEh (4) L+M l hillistone Nuclear Power Stetion Unit 3 of si of of op l213 1lOFl015 I lilLE 14) Potential Damage to Safety Related Eouipment Due to Design inadeouaev EVENT C# ATE (bl (EA NUMpEA t$) rstpOAT DATFI? OTMF A F A0ttreit $ NvotvED se t MONTP- DAY YEAR YEAR A MONTH DAY YEAR fA WIYNAMES j ofsiofofof I I i i 

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On November 15,1986 at 1630 hours, with the plant in hiode 1 at 100Fe power, engineering postulated a scenario which could, in the extreme case, result in a loss of redundant trains of safety related (vital) equipment. It was discovered that certam circumstances could lead to hiillstone Unit 3 becoming isolated from the hiillstone Station Switchyard while on-line. This cotid lead to an out-of-phase fast transfer to the Reserve Station Service Transformer (RSST) resulung in a potentially damaging transient on both trains of vital 4160V busses. On December 29, 1968, with the plant in hiode 1 at 75Fe power, three relays were identified that, assuming a single failure, also could result in the postulated scenario. Root cause of both events is inadequacy in plant design. The out-of-phase transfer scenario postulated had not been previously identified. As a resuh of the first event, administrative controls were implemented to realign the 4160V busses to the RSST when either of the switchyard breakers were open. As a result of the later esent, the power supply to the 4160V busses was manually abgned to the RSST. W)th the 4160V busses supphed by the RSST, the sceriatio is no longer feasible since the busses are in the post fast transfer state. A modification was l completed on June 22,1989 to ehmmate the last transfer on undervoltage. I 

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FACILITY NAME (1) DoCCT NVMBER '2l tFo N W PrA <ti FACsE 138 YEAR E Millstone Nuclear Power Station U"" 3 ~ ~ 0l6l0l0l0l4l2l3 $l8 0l 2l6 0l3 0l 2 OF 0l5 TEXT tit more space is roos'**c. use tiocition.' NRc Form 3%A e) (17) 

  ..        1.         Decenntion of Event j-On November 18,1966 at 1630 hours, with the plant in Mocie 1 at 100% power, plant engineering
  ,                   reported that a scenario had been postulated that could, in the extreme case, result in a loss of redundant trains of safety related (vital) equipment. This scenario was discovered during an engineering review of the effects of a main generator coastdown. Immediate action included the implementation of administrative controls to reduce the probability of occurrence of the postulated scenario. In addition, engineenng initiated a Significant Safety Hazard Evaluauon in compliance with 10CFR21. " Reporting of Defects and Noncomphance" The following is a description of the events sequence that could lead to a loss of redundant safety related equipment (refer to Figure 1 for the one-hne configuration of offsite and onsite power for Millstone Unit 3).

With Millstone Unit 3 main generator on-hne, this scenario can be initiated in one of two ways. f

1. Switchyard breaker 150-14T-2 is open or out of service for repairs and breaker 150-13T-2 opens automatically to isolate a fauh on the switchyard north bus.
2. Switchyard breaker 15G-13T-2 is open or out of service for repairs and breaker 150-14T-2 opens automatically to isolate a fault on the Millstone-Southington 348 transmission kne.

Both situation 1 and 2 result in the isolation of the main generator from the Millstone Station Switchyard. Under these circumstances, the main generator breaker would remain closed. The turbine subsequently tnps due to a power mismatch or turbine overspeed. A turbine-generator coastdown would follow. Since the main generator breaker receives no signal to open and excitation is still present, the main generator continues to provide power via the Normal Station Service Transformers (NSST's) to the Unit 3 6900V and 4160V electrical distribution systems. Based on a computer model of the generator coastdown, voltage on the 4160V busses would decay to 3220V at approximately 76 seconds after the turbine tnp. At this time the model predicts that the main generator frequency would be approximately 40HZ.. At 3220 volts, the undervoltage relays on non-vital busses 34 A and 34B actuate. After a 2 second time delay, all motor loads off of 34A and 34B tnp and also the supply breakers from NSST-A to bus 34A and 34B trip. The tripping of the norma) supply breaker irutiates a " fast transfer" to the Reserve Stauon Service Transformer " A" (RSST-A). Dunns this fast transfer, the RSST-A supply breakers to the vital busses 34C and 34D close approximately 6 cycles after the NSST-A supply breakers open. Due to the fact that motor loads remain connected to vital busses 34C and 34D during this 6 cycle interval, voltage /frequeracy on the busses is not expected to change significantly. Therefore, the RSST supply at 4160V/60Hz will not match the vital bus conditions of 3:20V/.10Hr. Vital bus connected loads will remain connected unless the fast transfer is not successful. Thus, the transient could damage the connected vital loads before the protective relays open RSST to 34C and 34D bus breakers. l l l l l l l NRO Form 366A (6-89)

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l. Deciminn nr Event Cont'd.  !

On December 29,1988 at 1600 hours, with the plant operating in Mode 1 at 75% power, 585 degrees F. 2250 psia, it was determined that a single failure of any one of three relays located at , the Millstone Station Switchyard could also cause this turbine-generator trip /coastdown and subsequent out-of-phase 4160V bus supply transfer. This determination was made dunng a Switchyard Electrical Protection Drawing review resulting from the irutial discovery of the postulated scenario. These relays of concern are the Pilot Wire Differential Relay 87PWY/U3, Pilot Wire Monitor and Transfer Tnp Relay 36PWY/U3, and the Pilot Wire Tnpping Relay 94PWY/U3. j At 1617 on December 29. 1968, the power supply to the 4160V busses was manually transferred to the RSST. In this configuration the bus transfer scheme is defeated, thereby chminating the possibihty of an out-of-phase transfer. 

            !!.        Cause of Event The root cause of these events is design inadequacy. The turbine generator coastdown/out-of-phase transfer scenario is an event which was not considered in the original design basis of the plant. Therefore, design of systems and circuitry to prevent such an event is not in compliance with apphcable Regulatory Guides or 10CFR des 1Fn criteria.

Ill. AnMysis of Event These events are being reported in accordance with 10CFR50.73(a)(2)(v'), as a discovery of a design inadequacy which alone could have prevented the fulfillment of the safety function of r structures or systems that are needed to shutdown the reactor and sustain it in a safe shutdown condition, remove residual heat, control the release of radioacuve material, or miugate the consequences of an accident. The postulated scenario could, in the extreme case, result in an overvoltage/overcurrent transient that could cause damage to the connected loads of the vital (safety related) bus. It can be l considered a common mode failure in that both trains of operating saftty related equipment would be simultaneously subjected to the transient. Although the fast transfer schemes are similar, the non-vital 6900V busses would not oe exposed to this type of transient because the undervoltage triays do not initiate the opening of their supply breakers from NSST-B. The undervoltage relays would shed all loads ! rom the busses, so the motors would not be subject to the undervoltage/underfrequency condition experienced while the busses remain connected to the generator dunng coastdown. Control room operators would have the option to manually transfer the 6900V feed over to the RSST-B dunng the coastdown without the threat of equipment damage. Pertaining to the November 16,1988 event, a seview of the Millstone Station Switchyard Log revealed that there were 12 instances where breaker 150-13T-2 m 150-14T-2 was open with , Millstone Unit 3 on-line. The durations vary in length from 7 minutes to 790 minutes, with 10 less , tnan 40 minutes. These were all due to manual switching evolutions (i.e., the breakers never opened automatically). I For the December 19,1988 even., the only failure mode of the specihed relays which could initiate l the postulated scenario is spontaneous operatica. The relays, which are normally deenergized, l would have to expenence a change of state due to shock or a susmic event. Since the relays are l not Class IE and have not been 6eismically tested, the profile of parameters required to cause this change of state is u*iknown. NAC Form 366A (6-89) t I

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OF ol6lolojol4l2l3 Fl6 0l 2l6 0l3 0l 4 0l5 T ext m mo.. . .e. .. ,.asir.o. u.. . anion i Nac .o m 386A .3 im I [. IV. CorrecWe Action j immediate corrective action included placement of caution tags on the Main Control Board status indicator lights for the 15G-13T-2 and 150-14T-2 swnchyard breakers. The tags require control room operators to manually transfer the Unit's offsite power supply to the Reserve Station Service Transformer, in the event that either of the two switchyard breakers open for any reason. When 3 the vital husses are supplied via the RSST's, the Fast Transfer Scheme is disabled, thus effectively preventmp the possibihty of this scenario from occurring. Engineennp reviewed possible scenarios with station power supplied from the RSST's and determined that it would not have a negauve impact on plant operations. This is based on the fact that a Loss of Power (LOP) event will still result in a transfer to the Emergency Diesel Generators, and, if required, the operators can manually switch back to the NSST's. On January 12, 1969, a temporary circuit modification was made which prevents the out-of-phase transfer from occurring. The modtheation removed the undervoltage trip of the NSST supply , breakers to the non-vital busses, effecuvely preventing a " fast transfer" on undervoltage. All other i aspects of the fast transfer were unaffected by the modification. The postulated scenario (or any i loss of vohage at the NSST) will now result in a

  • slow transfer", where the vital busses isolate from the non-vital busses and become connected to the RSST (if RSS'I voltape is greater than 90G) or Emergency Diesel Generators. This interma rnodificauon permitted the plant to return to the normal electrical configuration with the NSST supplying all 4160V loads.

On June 22, 1989, a permanent modification was completed to shminate the fast transfer on an undervoltage condition. This change employed a contact from the relay that receives the signal from the non-vital undervoltage relays to block the high speed closure of the supply breaker from the RSST. All other aspects of the fast transfer scheme remain unchanged. The fast transfer will still function as designed whenever the NSST supply breaker opens automatically for reasons other than undervoltage (i.e., overcurrent, current differential, etc.). An undervoltage condition on the non-vital busses will result in a slow tr msfer, where the vital to non-ntal bus ue breaker opens and the supply breaker from the RSST closes. In addition to the above, a modification was made to ensure that whenever switchyard breakers 250-13T-2 and 150-14T-2 art both open, the main generator output breaker and NSST supply breakers (both 4160V and 6960V) will trip. This will result in a 4160V and 6900V bus supply fast transfer to the RSST. V. Additional informntir n l LER 50-423/86-026 reports a plant tnp related to Emergency Diesel Generator Surveillance testing with the 4160V busses supplied by the RSST. The B Emergency Diesel Generator was started for surveillance testing. When the B Emergency Diesel Generator was paralleled to the emergency 4160V bus 34D, the normal to emergency bus tie breaker opened on reverse overcurrent trip, deenergizing the normal bus 34B. The cause of this event was procedural dehciency. The apphcable operatmg procedures have been corrected. EIIS CODES Synems Comnonents Switchyard System - FK Inter-Tie Transformers (NSST & RSST) - XFMR Switchyard Breakers - BKR j Relay - RLY 

        . N4c Form 366A (6a 59)

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